Wax has a reasonably high energy density and unlike other fuels it does not spill (when cold): can you make an adequate DIY stove with it?

Introduction

Wax is a hydrocarbon fuel with an energy density (by weight) comparable to other fuels like canister gas and white gas. It is therefore very similar in some ways to other fuels used in our little stoves, but it has the advantage that it is a normally solid: it won't spill. Small tea-tray candles have been used for some time to keep food warm, but they are a bit too small to bring water to the boil in a reasonable time. A problem with just scaling a tea candle up is the soot generated. We explore here what can be done to improve the heating capacity and reduce the soot from a 'candle stove'.

All the experimental work detailed here was done by Forum Reader Mark Hurd. Backpacking Light Senior Editor Roger Caffin offered some advice and wrote the article text, and accepts responsibility for any errors.

What's Good

Robust, won't spill

Can be refuelled while running

Very cheap to make

Stable on the ground

What's Not So Good

Low in power

Generates considerable soot

Generates acrid fumes while burning some waxes

Generates lots of fumes after being extinguished

Hard to light

Background Discussion

First of all, it is obvious that candle wax burns and makes heat. Small flat candles have long been sold and used to heat 'chafing dishes', and are often known as 'tea candles'. But candle wax has one advantage over most other fuels except for those in the Hexamine class: cold wax is a solid and does not spill or leak in your pack. Compared to Hexamine-class fuels it appears to have extremely low toxicity as well. So for many situations it looks as though it might have some potential.

Just to reinforce this, we list here some properties of some the more common fuels we use, including two forms of wax - beeswax and paraffin. Be assured - there are many others.

Fuel

Formula

Kcal/g

BTU/lb

Propane

C3

11.0

21660

Beeswax

C26-46

11.0

21650

Butane

C4

10.8

21160

Gasoline

C4-12

~10.4

~18500

Kerosene

C10-18

~10.3

~18300

Paraffin

C19-36

10.0

18000

Hexamine

7.3

12930

Ethanol

CH3CH2OH

6.4

12760

Methanol

CH3OH

4.7

9750

Dry wood

~3.9

~7000

A candle stove with a cross-wick and a mesh pot-support

Surprise: beeswax and paraffin wax are up there with the best! But in fact that is quite reasonable because they are just higher hydrocarbons, in the same general series as propane, butane, gasoline and kerosene. Incidentally, you should treat many of the values given here with some caution as the composition of many of the fuels can vary. (Just what is in gasoline anyhow?)

But it is also known that candles do have some problems: typical candles have very small flames and make soot. Can these problems be overcome and a more powerful candle stove created?

A coiled-up wick made of corrugated cardboard - it would be very sooty IF you could get it burning

Flame Design

You might think that this section should be headed 'wick design', but that would be to miss the point. What we are concerned about is the flame, not the wick. We want a flame large enough to heat water at an acceptable rate, while not emitting too much soot. Sadly, at this stage it would seem that some soot is going to be inevitable, so we will try to minimise it.

One way of increasing the flame size is to make a bigger wick. In the photo to the right we show a 'coil wick' (picture courtesy zenstoves.net). This is a big coil of corrugated carboard embedded in the wax. In fact the coil almost fills the tin, even though it can be hard to see it all under the wax. This design is not going to work very well at all. For a start, getting it lit may prove almost impossible unless you use a small blow-torch to melt enough of the wax that the wick can light. Even if we assume a slight modification so the wick can be lit, it may make a large flame but it will also generate a huge amount of soot and lots of fumes too. This is because the centre region of the wick will be releasing lots of wax vapour but this vapour won't get to burn very well: the outer wall of flame will effectively prevent any oxygen from reaching it. No oxygen means no combustion, but lots of soot (and maybe a bit of carbon monoxide as well).

A newly cast stove with a cardboard Cross-Wick

The flame needs to have lots of sidewall where oxygen can mix in to support combustion. And both sides of the flame need to be open to the air: a simple ring will still leave the core without oxygen. This makes for poorer combustion and probably more fumes and carbon monoxide. So instead of a ring we look at a cross arrangement. The centre region of a cross is likely to be oxygen poor so we eliminate it. The result looks like the stove shown to the left.

Here we have a new candle stove cooling down from the casting process: the central pool of wax is still hot and clear. There are effectively four wide flat wicks here. The flat design of the wick means that the flame will be flat and will get as much air as possible. Having the four wicks out at the edge means there is minimal oxygen starvation in the centre. The four-wicks design is simple and possibly adequate in a can of this size: a larger can could have more wicks - perhaps.

How big a gap should be left in the centre of the wick? That is hard to say, and has to be judged from the flame pattern. The flames from this example may be seen in the first photo. They are perhaps a little high and smokey, but that seems inevitable with candle stoves.

Other designs are also possible: a tubular wick as shown below to the left has been tried and is called a Circle Wick. It is a simple circle of wick, as opposed to the coil wick shown before. The wick was made quite high to get good power, but there are costs to this as explained below. A Spiral Wick consisting of round 'wick' cord wound around the side of a light tube has also been tried, as shown to the right. This was meant to be similar to some alcohol stoves, but the results were not so good.

A high Circle Wick stove, burning (needs mesh pot support)

Spiral Wick stove, with integral pot support

Testing the Efficiency

The testing discussed here and in also the Carbon Monoxide series of articles, plus the basic chemistry of combustion, indicate that having enough air supply to the flames is vital to getting the maximum heat from the fuel. So the obvious question is how can the air supply be maximised? In addition, note that maximising the air supply to the flame should minimise the amount of soot and fumes produced. Reducing the amount of soot is clearly desirable just from the convenience point of view: consider the deposit on the bottom of this pot after boiling half a litre (one pint) of water on the stove arrangement shown above.

Soot on the underside of a pot

But remember that the soot deposited on the pot is actually unburnt carbon from the fuel: it is wasted fuel and wasted energy. In fact it is the carbon in the fuel which supplies most of the energy (as opposed to the hydrogen), so the loss of power is quite significant. If all that carbon had been fully burnt in the flame instead landing on the pot as shown the stove would have given out more heat per gram of fuel. So soot is very bad for a number of reasons.

It is likely that fuel consumption will be a good guide to the efficiency of the air supply, so the three wick designs illustrated above were tested a number of times in various configurations. For all the tests results shown here the fuel used was beeswax. The reason for using beeswax is given later. The test conditions were the same for all three designs, thus:

Air Temperature

75 F / 24 C

Starting water temperature

60 F / 15.5 C

Finish temperature (boiling)

212 F / 100 C

Volume of water

16 fl oz / 455 ml

Pot

1.3 liter Evernew Ti

The averaged results from several trials of each design using beeswax are as follows.

Design

Wax used

Boil Times

Best Clearance

Cross-Wick

6 g

16:00 min

Not measured

Circle-Wick

10.5 g

6:10

1.5 cm

Spiral-Wick

11 g

8:30

1.0 cm

Clearly the increased air supply to the Cross-Wick design means that a lot more energy was extracted from the fuel, compared to the two very inefficient round wicks where the air supply to the inside of the flame varied from very poor to non-existent. However, the Cross-Wick design was a lot slower to bring the water to the boil compared with the other two. This happened despite the probable loss of some flame energy up the sides of the pot with the Circle-Wick and the Spiral-Wick designs. We believe that this is an inevitable result of wanting to get the best possible air supply, given a finite stove area. That is, the amount of flame surface area was smaller for the cross-wick design compared to the other two.

The Circle-Wick design allowed the height of the wick to be varied easily - with a pair of scissors. It was found that wicks of 1.5 cm and 2 cm length above the rim of the container gave about the same performance, with average boil times 6:18 min and 6:10 min respectively. A wick height of 2 cm did give a boil time of 5:58 min on one occasion. Reducing the height of the wick to 1 cm above the rim gave an 8+ min boil time. The wick to pot distance was kept constant at 1.5 cm for the rest of these tests.

The performance of each stove is fairly sensitive to the height of the pot above the top of the wick. Reducing the height from 1.5 cm to 1 cm seriously extended the time to boil - in some cases to beyond the patience of the Tester. This is explained by the reduced availability of air for combustion and the much poorer flame which resulted.

Operational Considerations

All of these stoves produce a lot of wax fumes. The fumes are very unpleasant and permeate everything around. They are of course a by-product of the boiling wax. An attempt was made to create a design which would allow the fumes to be burnt, but no suitable design was found during these tests. This may leave opportunities for further experiments by someone else.

The paraffin wax put out acrid eye and throat burning fumes. This does not happen with an ordinary candle, but then, an ordinary candle does not have a flame of the size seen here. The beeswax fumes are somewhat acrid, but mostly cloying. Beeswax candles have always been favoured by the more affluent in times past, and this is also why beeswax rather than paraffin wax was used for the tests reported above. However, beeswax apparently has a reputation of attracting bears: residual organics from the honey vaporising from the wax is one reason. Just the smell of the beeswax stove in your pack might also be enough to attract bears too: they have a very sensitive sense of smell.

A snuffed stove emitting lots of fumes

As mentioned above, in general the stoves made a sooty mess of the pots, and if the soot included some wax condensate it was very hard to clean the pots afterwards. And of course the stuff makes black marks on anything it touches: hands, clothing, gear ...

When making these stoves it is important to make sure the wick is saturated with wax. A dry wick is obviously not going to catch alight at all. But a fully impregnated wick has a fair thermal mass, which means that the stoves are all hard to light, especially the Spiral-Wick design. That one can takes several minutes of flame from a lighter to catch. This is because the flame from the lighter has to melt and then vaporise some of the wax before it can catch alight. The problem can be reduced if a thread or two from the wick is left sticking out from the main wick - provided it too has been impregnated with wax as well. If you do this you have something very similar to an ordinary candle wick.

Extinguishing one of these candle stoves is hard. The Cross-Wick design can be blown out - just, but the other two cannot be blown out. This means they have to be snuffed out. But while this extinguishes the flame (by removing the air supply), it does not stop the emission of vapour/fumes from the hot wax: that still goes on for several minutes. Now there is no flame the amount of fumes is of course even worse than before. The result can be a rather thick fog as shown here, and the fumes get into everything around the stove. In addition, the stove stays very hot for up to 10+ minutes, during which time the liquid wax can be spilt.

Material Sources

Waxes can be bought from most any craft shop and from the web, often by the pound. Sources are not given here.

The first wicks in the experiments were simply corrugated cardboard: as cheap and as simple as you can get. In fact they worked quite well, but they do have the problem that as the wax gets low the wick starts to burn. This makes a bit of extra smoke, which is not so nice. More sophisticated wicks use non-burning synthetics, although one could use cheap cotton tape as well (but it chars like the paper). Sources for synthetic wicks include:

wickstore.com -- Lots of different kind of wicks including fiberglass. They sell it by the 100+ yard spool, but more importantly they also sell 10 yard hanks for US$7 each.
Spiral-Wick stoves were made with 1/8 inch and 1/4 inch round braided fiberglass wick, by wrapping the round wick material around a small cat food can, placing the wrapped can in the center of a large cat food can and pouring molten wax in the space between.

www.flammaaeterna.com (suggested by Forum Reader Jason Klass) -- This site caters to "fire-oriented performance art", such as dancing while juggling flaming objects (er - wow!). They sell flat Kevlar/Fiberglass tape wicks in various thicknesses and widths up to 4 inches. (As an aside: these wicks also make good insulators for pots and pot handles, which is what Jason was using it for.)
Two sizes of the K1 Tape Wick, 1/8 inch by 2.5 inches and 1/16 inch by 3 inches, were used for the Circle-Wick stoves. They are stiff enough to stand up by themselves. A length of the tape wick long enough to go around the inside circumference of a small cat food can was placed in position and molten wax was then poured into the can. The wick may be cut to the desired height above the rim, but should be soaked in wax to the top. Cutting the top edge of the wick leaves a raw edge on top of the wick which should be easier to light.

> needs some tuning, the flame is a bit stronger than I would have liked because I think it has too much thermal mass.I will disagree here. I don't think the thermal mass has anything to do with the amount of flame. If there is too much flame it means too much alcohol vapour is coming off. You could reduce this by getting the flame further away from the metal (less boiling), or by covering the top ring and using jets to restrict the flow of vapour.

I know you have been given at least one place to look for JB Weld in Australia so this may not be necessary.

When you posted your question I sent an email to the New Zealand distributor for JB Weld to ask where it was sold in Australia. I had exchanged emails with this person a few years ago. I just got an answer back from them and thought I would still pass it on to you. This is what they sent:====Contact the company below who are the Australian distributor. They should be able to help you.

I would like to test the Primus ETA pots and I have checked them out on the net but I have not seen them for sale yet in OZ, also my stove/pot testing is totally self funded, I will look at testing Primus ETA pots when funds become available. For a while my nephew worked at an outdoor shop and he got me some stoves including the JetBoil stove and pots at a good price but he has now moved on to another job and I have to pay near retail, prices are generally much higher here than the US.

"I will disagree here. I don't think the thermal mass has anything to do with the amount of flame. If there is too much flame it means too much alcohol vapour is coming off. You could reduce this by getting the flame further away from the metal (less boiling), or by covering the top ring and using jets to restrict the flow of vapour."

You might be right about the moving the flame further away but restricting the flow of vapor did not work with this stove, I have tried using jets (see picture below) and the stove used its fuel even quicker than the open top. I was trying to design an inside out Gram Weenie with a central flame to improve efficiency.

As part of another project I have been building a similar alcohol stove with the very thin aluminium Red Bull cans and I have had the opposite problem of not been able to get a strong enough flame, I moved the flame closer and I now have a nice flame.

Thanks everyone for the response.I'll ask a couple of mates who are mechanics if they can score me anything, and if that fails it sounds like I should head into KMart.I went to Bunnings about 18months ago after JB weld and that 3M silver tape that is uber-heat resistant and they couldn't help me on that front. After contacting 3M (Aus) directly, they sent me a sample of a fibreglass tape which works ok, but sometimes catches on fire momentarily!

I can't stand the smoke and fumes emitted after a candle is blown out, as it seems to hang in the air forever. Then I noticed that the wick glows long after the flame is gone, and the gasified wax continues to stream off the whole time. I discovered that by touching a bit of the liquified wax to the tip of the wick immediately put an end to the smoke! So I'm thinking that the same technique could be used on the much larger wicks found on these candle stoves, maybe using a small flux aplicator brush. What to do about soot accumulation would be more of a problem for me.

To eliminate fumes when extinguishing a candle I just wet my thumb and index fingers and put the flame out with them. (try it if you have never done this) So maybe this can be extended to multiple flames by using a cap that has some wet fabric inside or something like that.Franco

Ok first let me say I have used candles to cook with before. I learnt to in the military because we used candles to heat up positions in the winter. I added a canteen cup to get something hot to drink. It can be done, carefully. Secondly, I learnt from some of the older guys that if you rub soap on the cooking pot before putting it over the candle, the soot collects on the soap. Than when you clean, wash the soap off and the soot goes away easily. Please use biodegradable soap. Thirdly, if the wax is heated slightly first than used in liguid form it will operate better. How is much like a rocket engine. The wax is placed in an outer container. A inner container it where the liquid wax comes to be burned. Holes in the inner container allow the wax to flow into the center. As the melted wax burns it preheats the outer containter and allows more wax to melt. One problem, keeping the outer container from catching fire. I have some designs, not avaible tonight, but most involve simple sealing air out of the outer container. Mike

I would love to see your designs when you get around to it. I looked at the 2 chamber idea and saw a prototype on the web from a student at the Univ. of Johannesburg, but the link is broken now. His stove used wax chips in a hopper like area that slowly fed into the melted wax. I'm afraid my experiments were far too rudimentary.

The trick with the soap I remember from my wood fire days, but it somehow didn't occur to me with these stoves.

Thanks for your input and I'm looking forward to any designs you might post.

Revisitng the idea of candle stoves, would anyone happen to know the energy density of vaseline/petroleum jelly? Can't seem to find it online. And does it burn any cleaner than wax?

I ran a small test and a tiny amount of vaseline in a beer bottle top, with a small wad of cotton wool smeared with vaseline as both wick and firestarter, lit with a firesteel, burned for a long time and threw off a lot of heat. It didn't seem too sooty either, though there was just the one flame of course. It would seem to beat a candle stove in at least one respect: that of being extremely easy to light with just a firesteel.

With the area of the wad being fairly large in comparison to the size of the bottle top, you get quite a wide flame as well, comparatively speaking, though the height was fairly small - comparable to a normal candle rather than to an alcohol stove. In a normal tealight candle, the flame width is limited by the thin wick, which I would guess reduces the burn rate of the fuel and lengthens the time you wait for a brew.

I'd imagine (with the benefit of ignorance) there's some optimal size of flat, round wick (like part of a cotton make-up removal pad, say) that would increase the amount of vaseline being burned whilst balancing that against the need for the limited supply of air to give clean combustion of the fuel.